Years ago, during my school years and my first employment years, I quickly realized an emerging pattern: when it comes to computing I'm generally able to leverage knowledge that I had previously gathered on my spare time. All this computer-related stuff I read on my spare time, all these languages I try for no apparent reason, that generally pays off sooner or later.
I was learning Pascal on my spare time when Basic was a requirement at school and I already knew quite well. Then at the university, I was learning C and Objective-Pascal when classes were done in Pascal. Then I started exploring C++ just to find out that's what needed for work, and so forth with PHP, .Net, C#, Perl and more recently Python.

Back in 2001 I attended the first Mozilla Developer Day at the Netscape's headquarters, followed by the first Mozilla party. At more or less the same time I was checking out Mozilla and fighting to have it build and eventually succeeded -- a useless exercise as I was not really planning to contribute to it. Merely out of curiosity and totally worth it, if not just to realize that there was a daunting amount of code involved and pretty much an equal lack of documentation.

Two years ago I received the book Rapid Application Development with Mozilla which triggered the same behavior -- checkout the code, compile it, browse it and move on to something else. At least I can understand a sentence that combines lots of acronyms like XUL, XBL, XPCOM and Gecko and feel like I know what the concepts are all about.

All these little efforts seemed basically like useless exercises at the time. Yet, in much accordance to the above mentioned pattern, this is going to prove useful sooner than later.

More recently I heard of Dive Into Greasemonkey and this may as well become a good read.